Building Soils for Better Crops, 2nd Edition

	Reducing Tillage the crying need is for a soil surface similar to that which we find in nature. [and] the way to attain it is to use an implement that is incapable of burying the trash it encounters; in other words, any implement except the plow. E.H. Faulkner, 1943 Although tillage is an ancient practice, the question of which tillage system is most appropriate for any particular field or farm is still difficult to answer. Before we discuss different tillage systems, let's consider why people started tilling ground. Intensive, full-field tillage was first practiced by farmers who grew small-grain crops, such as wheat, rye and barley, primarily in Western Asia, Europe, and Northern Africa. Tillage was needed to control weeds and give the crop a head-start before a new flush of weeds germinated. It also stimulated mineralization of organic forms of nitrogen to forms that plants could use. Mostly, however, intensive tillage created a fine seedbed, thereby greatly improving germination. The soil was typically loosened by plowing and then dragged to pulverize the clods and create a finely aggregated and smooth seedbed. The loosened soil also tended to provide a more favorable rooting environment, facilitating seedling survival and plant growth. From early on, animal traction was employed to accomplish this arduous task. At the end of the growing season, the entire crop was harvested, because the straw also had considerable economic value for animal bedding, roofing thatch, and brick making. Sometimes, fields were burned after crop harvest to remove remaining crop residues and to control pests. Although this tillage-cropping system lasted for a long time, it resulted in excessive erosion, especially in the Mediterranean region, where it caused extensive soil degradation. Eventually deserts spread as the climate became drier. Other ancient agricultural systems, notably those in the Americas, did not use intensive full-field tillage for grain production. Instead, they used a hoe for manual tillage that created small mounds (hilling). This was well adapted to the regional staples of corn and beans, which have larger seeds and require lower planting densities than wheat, rye, and barley. Several seeds were placed in a small hill, often with the help of a planting stick, and hills were spaced several feet apart. In many, but not all cases, the hills were elevated to provide a temperature and moisture advantage to the crop. Compared with the cereal-based systems growing only one crop in a monoculture, these fields often included two or three plant species growing at the same time. This hilling system was generally less prone to erosion than whole-field tillage, but climate and soil conditions on steep slopes still frequently caused considerable soil degradation. A third tillage system was practiced as part of the rice-growing cultures in southern and eastern Asia. Here, paddies were tilled to control weeds, puddle the soil, and create a dense layer that limited the downward losses of water through the soil. The puddling process occurred when the soil was worked while wet in the plastic or liquid consistency state and was specifically aimed at destroying soil structure. This system was designed because rice plants thrive under flooded conditions. There is little soil erosion, because paddy rice must be grown either on flat or terraced lands and runoff is controlled as part of the process of growing the crop. Full-field tillage systems became more widespread as the influence of European culture expanded into other regions of the world. It's better adapted to mechanized agriculture so the traditional "hill crops" eventually became row crops. The invention of the moldboard plow provided a more effective tool for weed control by fully turning under crop residues, growing weeds, and weed seeds. The development of increasingly powerful and comfortable tractors made tillage an easier task. In fact, it has become almost a recreational activity for some farmers. New technologies have lessened the need for tillage. The development of herbicides reduced the need for tillage as a weed control method. New planters achieved better seed placement, even without preparing a seedbed beforehand. Amendments, such as fertilizers and liquid manures, can be directly injected or band-applied. Now, there are even vegetable transplanters that provide good soil-root contact in reduced or no-till systems. Although herbicides often are used to kill cover crops before planting the main crop, farmers and researchers have found that they can obtain fairly good cover crop control through well-timed mowing, rolling, or rolling/chopping greatly reducing the amount of herbicide needed. Technologies have lessened the need for tillage herbicides new planters and transplanters new physical methods for cover crop suppression Increased mechanization, intensive tillage, and erosion have degraded many agricultural soils to such an extent that people think they require tillage to provide temporary relief from compaction. As aggregates are destroyed, crusting and compaction create a soil "addicted" to tillage. Except perhaps for organic crop production systems, where tillage is needed because herbicides aren't used, a crop can be produced with limited or no tillage with the same economic return as conventional tillage systems. Managing soil in the right way to make reduced tillage systems successful, however, remains a considerable challenge. Tillage Systems Tillage systems are often classified by the amount of surface residue left on the soil surface. Conservation tillage systems are those that leave more than 30 percent of the soil surface covered with crop residue. This is considered to be a level at which erosion is significantly reduced (see figure 15.1). Of course, this partially depends on the amount of residue left after harvest, which may vary greatly among crops and harvest method (for example, corn harvested for grain or silage). Although surface residue cover greatly influences erosion potential, the sole focus on it is somewhat misleading. Erosion potential also is affected by factors such as surface roughness and soil loosening. Another distinction of tillage systems is whether they are full-field systems or restricted tillage systems (figure 15.2). The benefits and limitations of various tillage systems are compared in table 15.1. Figure 15.1 Soil erosion dramatically decreases with increasing surface cover. (Fall plow (FP), fall chisel (FC), no-till (NT), corn = circles, soybeans = no circles). Modified from Manuring, 1979. Conventional Tillage A full-field system manages the soil uniformly across the entire field surface. It typically involves a primary pass to loosen the soil and incorporate materials at the surface (fertilizers, amendments, weeds, etc.), followed by one or more secondary tillage passes to create a suitable seedbed. Primary tillage tools are generally moldboard plows, chisels, and disks, while secondary tillage is accomplished with finishing disks, tine or tooth harrows, rollers, packers, drags, etc. These tillage systems create a uniform and often finely aggregated seedbed over the entire surface of the field. Such systems appear to perform well because they create near-ideal conditions for seed germination and crop establishment. a b c d Figure 15.2 Four tillage systems. a) Chisel tillage: Shanks provide full-field soil loosening. b) No-till: Corn was directly planted into untilled soil. Photo by NRCS. c) Zone tillage: Planter loosens soil in the row and moves residues to the side. d) Ridge tillage: Crop is planted into small ridges without tillage. Moldboard plowing is generally the least desirable practice because it is energy intensive, leaves very little residue on the surface, and often requires multiple secondary tillage passes. It also tends to create plow pans. However, it is generally the most reliable practice and almost always results in reasonable crop growth. Chisel implements generally provide results similar to the moldboard plow, but require less energy and leave significantly more residue on the surface. Chisels also allow for more flexibility in depth of tillage, generally from 5 to 12 inches, with some tools specifically designed to go deeper. Disks usually perform shallow tillage, depending on their size, and still leave residue on the surface. They can be used as both primary and secondary tillage tools. Although full-field tillage systems have their disadvantages, they often can help overcome certain problems, such as compaction and high weed pressures. Organic farmers often use moldboard plowing as a necessity to provide adequate weed control and facilitate nitrogen release from incorporated legumes. Livestock-based farms often use a plow to incorporate manure and to help make rotation transitions from sod crops to row crops.   Besides incorporating surface residue, full-field tillage systems with intensive secondary tillage crush the natural soil aggregates. The pulverized soil does not take heavy rainfall well. The lack of surface residue causes sealing at the surface, which generates runoff and erosion and creates hard crusts after drying. Also, intensively tilled soil will settle after some rainfall and may "hardset" upon drying, thereby restricting root growth. Full-field tillage systems can be improved by using tools, such as chisels (figure 15.2a), that leave some residue on the surface. Reducing secondary tillage also helps decrease negative aspects of full-field tillage. Compacted soils tend to till up cloddy and intensive harrowing and packing is then seen as necessary to create a good seedbed. This creates a vicious cycle of further soil degradation with intensive tillage. Secondary tillage often can be reduced through the use of state-of-the-art planters, which create a finely aggregated zone around the seed without requiring the entire soil width to be pulverized. Indeed, a good planter is perhaps the most important secondary tillage tool, because it helps overcome poor soil-seed contact without destroying surface aggregates over the entire field. A fringe benefit of reduced secondary tillage is that rougher soil has much higher water infiltration rates and reduces problems with settling and hardsetting after rains. Weed seed germination is also generally reduced, but pre-emergence herbicides tend to be less effective than with smooth seedbeds. Reducing secondary tillage may, therefore, require emphasis on post-emergence weed control. Restricted Tillage Systems These systems are based on the idea that tillage can be limited to the area around the plant row and does not have to disturb the entire field. Three tillage systems fit this concept no-till, zone-till, and ridge-till. The no-till system (figure 15.2b) does not involve any soil loosening, except for a very narrow and shallow area immediately around the seed zone. This localized disturbance is typically accomplished with a fluted, or ripple, coulter on a planter. This is the most extreme change from conventional tillage. No-till systems have been used successfully on many soils in different climates. They are especially well adapted to coarse-textured soils (sands and gravels), as they tend to be softer and less susceptible to compaction. It typically takes a few years for no-tilled soils to improve, after which no-till crops often out-yield crops grown with conventional tillage. The quality of no-tilled soils, by almost any measure, improves over time. The maintenance of surface residue protects against erosion and increases biological activity by protecting the soil from temperature and heat extremes. Surface residues also reduce water evaporation, which combined with deeper rooting reduces the susceptibility to drought. Another system, usually called zone tillage (figure 15.2c), gets some of the benefits of soil disturbance in the soil around the plant row without disturbing the entire field. It uses multiple fluted coulters mounted on the front of a planter (figure 15.3) to develop a fine seedbed of approximately 6 inches wide by 4 inches deep, and typically uses trash wheels to move residue away from the row. The system may also include a separate pass of a "zone building" implement during the off season (see figure 14.1). This typically involves a narrow shank or knife, sometimes used to inject fertilizers, combined with a trash remover or hilling disk (to perform in-row tillage and overcome compaction problems). The term "strip tillage" often is used to describe the latter system. Figure 15.3 Zone-till planter. a. Coulters (cut up residues and break up soil in seed zone); b. Ferti-lizer disc openers (place granular starter fertilizer in a band next to the seed); c. Spider (trash) wheels (move residue away from the row); d. Seed place-ment unit; e. Press wheels (create firm seedbed); and f. Wheel used for transporting the planter. Ridge tillage (figure 15.2d) combines some tillage with a ridging operation. This system is particularly attractive for cold and wet soils, because the ridges offer developing plants a warmer and better-drained environment. The ridging operation can be combined with mechanical weed control and allows for band application of herbicides. This decreases the cost of chemical weed control, allowing for about a two-thirds reduction in herbicide use. For fine and medium-textured soils, zone and ridge tillage systems generally work better than strict no-till, especially in regions with cold and wet springs. Because these soils are more susceptible to compaction, some soil disturbance is probably beneficial. No-till is used successfully for narrow-row crops, including small grains, perennial legumes and grasses. Zone and ridge tillage are only adapted to wide-row crops with 30-inch spacing or more. Which tillage system for your farm? This is difficult to answer. It depends on your climate, soils, cropping systems, and your objectives. Here are some general guidelines. Grain and vegetable farms have great flexibility with adopting reduced tillage systems. In the long run, limited disturbance and residue cover improve soil quality, reduce erosion, and boost yields. A negative aspect of these systems is that, at first, they may require more herbicides. However, combining reduced tillage with use of cover crops frequently helps reduce weed problems. Weed pressures typically decrease significantly after a few years. Mulched cover crops, as well as newly designed mechanical cultivators, help provide effective weed control in high-residue systems. Some innovative farmers use no-till combined with a cover crop, which is mowed or otherwise killed to create a thick mulch. Steve Groff, a vegetable and field crop farmer in Pennsylvania, modified a rolling stalk chopper to roll down and crimp his vetch/rye cover crop, providing weed control with minimal use of herbicides (see profile). Figure 15.4 No-till transplanting of vegetables into a killed cover crop on the Groff farm in Pennsylvania. Photo by Ray Weil. Farmers need to be aware of potential soil compaction problems with reduced tillage. If a strict no-till system is used on a compacted soil, especially on medium or fine-textured soils, serious yield reductions may occur. As discussed in chapter 6, dense soils have a narrow water range for which plant growth is not restricted. Crops growing on compacted soils are more susceptible to inadequate aeration during wet periods and restricted root growth and inadequate moisture during drier periods. Compaction, therefore, reduces plant growth and makes crops more susceptible to pest pressures. In poorly structured soils, tools like zone-builders and zone-till planters may provide compaction relief in the row, while maintaining an undisturbed soil surface. Over time, soil structure improves, unless recompaction occurs from other field operations. Crops grown on imperfectly drained soils tend to benefit greatly from ridging or bedding, because part of the root zone remains aerobic during wet periods. These systems also use controlled traffic lanes, which greatly reduce compaction problems. Unfortunately, matching wheel spacing and tire width for planting and harvesting equipment is not always an easy task. Before Converting to No-Till An Ohio farmer asked one of the authors what could be done about a compacted, low organic matter, and low fertility field that had been converted to no-till a few years before. Clearly, the soil's organic matter and nutrient levels should have been increased and the compaction alleviated before the change. Once you're committed to no-till, you've lost the opportunity to easily and rapidly change the soil's fertility or physical properties. The recommendation is really the same as for someone establishing a perennial crop like an apple orchard. Build up the soil and remedy compaction problems before converting to no-till. It's going to be much harder to do later on. Once in no-till, there are some things that can be tried to break up compacted soils, such as a sorghum-sudangrass cover crop. However, a severe compaction problem may require tilling up the soil and starting over. For organic farms, as with traditional farms before agrichemicals were available, full-field tillage may be necessary for mechanical weed control and incorporation of manures and composts. Organic farming on lands prone to erosion may, therefore, involve tradeoffs. Erosion can be reduced by using rotations with perennial crops and a modern planter to establish good crop stands without excessive secondary tillage. In addition, soil structure may be easier to maintain, because organic farms generally use more organic inputs, such as manures and composts. Livestock-based farms face special challenges related to applying manures or composts to the soil. Although these materials may sometimes be injected directly, some type of tillage usually is needed to avoid large losses of nitrogen by volatilization and phosphorus and pathogens by runoff. Transitions from sod to row crops are usually easier with some tillage. Farmers raising livestock should try to reduce tillage as much as possible and use methods that leave residue on the surface. Rotate Tillage Systems? A tillage program does not need to be rigid. When changing to reduced tillage, consider incorporating nutrients and organic matter with the moldboard plow (see box on p. 142). Fields that are zone or no-tilled may occasionally need a full-field tillage pass to provide compaction relief or to incorporate amendments. Tilling a no-till or zone-till field should be done only if clearly needed. Although a flexible tillage program offers a number of benefits, aggressive tillage with a moldboard plow and harrows on soils for which no-till is best adapted will destroy the favorable soil structure built up by years of no-till management. Timing of Field Operations The success of a tillage system depends on many other factors. For example, reduced tillage systems, especially in the early transition years, may require more attention to nitrogen management, as well as weed, insect, and disease control. Also, the performance of tillage systems may be affected by the timing of field operations. If tillage or planting is done when the soil is too wet (its water content is above the plastic limit) then cloddiness and poor seed placement may result in poor stands. Tillage also is not recommended when soil is too dry, because of excess dust creation, especially on compacted soils. A "ball test" (Chapter 6) helps ensure that field conditions are right. Frost Tillage? You may have heard of frost seeding legumes into a pasture, hayfield, or winter wheat crop in very early spring, but probably never heard of tilling a frozen soil. It seems a strange concept, but some farmers are using frost tillage as a way to be timely and reduce unintended tillage damage. It can be done after frost has first entered the soil, but before it has penetrated more than 4 inches. Water moves upward to the freezing front and the soil underneath dries. This makes it tillable as long as the frost layer is not too thick. Compaction is reduced because equipment is supported by the frozen layer. The resulting rough surface is favorable for water infiltration and runoff prevention. Some livestock farmers like frost tillage as a way to incorporate or inject manure in the winter.   Optimum Tillage System New agricultural technologies provide opportunities to reduce tillage and improve soil quality. The optimum system for any farm depends mainly on soils and climate, as well as the need for mechanical weed control, incorporation of cover crops and animal manures, and lessening compaction. Tillage systems should change in the direction of those that leave residue and mulches on the surface and that limit the pulverization of soil aggregates. Because soil compaction may affect the success of a reduced tillage system, a whole-system approach to soil management is needed. For example, no-till systems that also involve harvesting operations with heavy equipment will succeed only if traffic can be restricted to dry conditions or fixed lanes within the field. Even zone-tillage methods may fail if heavy harvest equipment is used without fixed lanes. Soils that are severely eroded and low in organic matter may need careful management when making the transition to reduced tillage systems. In such cases, methods that increase the soil organic matter content and improve soil structure (for example, cover cropping and organic amendments) before reducing tillage will improve the probability for success of these systems. As surface residue levels increase with the start of reduced tillage, some soil loosening may be needed to relieve compaction. Sources Cornell Recommendations for Integrated Field Crop Production. 2000. Cornell Cooperative Extension, Ithaca, NY. Manuring. 1979. Cooperative Extension Service Publication AY-222, Purdue University. West Lafayette, IN. Ontario Ministry of Agriculture, Food, and Rural Affairs. 1997. No-till: Making it Work. Available from the Ontario Federation of Agriculture, Toronto, Ontario (Canada). van Es, H.M., A.T. DeGaetano, and D.S. Wilks. 1998. Upscaling plot-based research information: Frost tillage. Nutrient Cycling in Agroecosystems. 50: 85­90.   Top